Convoy SC 121 was the 121st of the numbered series of World War II slow convoys of merchant ships from Sydney, Cape Breton Island to Liverpool. The ships departed New York City 23 February 1943; and were met by the Mid-Ocean Escort Force Group A-3 consisting of the United States Coast Guard (USCG) Treasury-class cutter USCGC Spencer, the American Wickes-class destroyer USS Greer, the British and Canadian Flower-class corvettes HMS Dianthus, HMCS Rosthern, HMCS Trillium and HMCS Dauphin and the convoy rescue ship Melrose Abbey. Three of the escorts had defective sonar and three had unserviceable radar.
Background
As western Atlantic coastal convoys brought an end to the second happy time, Admiral Karl Dönitz, the Befehlshaber der U-Boote (BdU) or commander in chief of U-Boats, shifted focus to the mid-Atlantic to avoid aircraft patrols. Although convoy routing was less predictable in the mid-ocean, Dönitz anticipated that the increased numbers of U-boats being produced would be able to find convoys with the advantage of intelligence gained through B-Dienst decryption of British Naval Cypher Number 3. Only 20 percent of the 180 trans-Atlantic convoys, from the end of July 1942 until the end of April 1943, lost ships to U-boat attack.
Battle
On 6 March U-405 sighted the convoy, which had been scattered by nine consecutive days of northwesterly Force 10 gales and snow squalls. The storm damaged the radio communication system aboard the escort commander's ship Spencer and Dauphin had to leave the convoy with damaged steering gear. U-230 torpedoed the British freighter Egyptian on the night of 6–7 March. The British freighter Empire Impala stopped to rescue survivors and was torpedoed after dawn by U-591.
U-190 torpedoed the British freighter Empire Lakeland when the gale subsided on 8 March and four more stragglers were sunk by U-526, U-527, U-591, and U-642. On 9 March the convoy escort was reinforced by No. 120 Squadron RAF B-24 Liberators from Northern Ireland and by the Wickes-class destroyer USS Babbitt and the Treasury-class cutters USCGC Bibb and USCGC Ingham from Iceland.
U-530 torpedoed straggling Swedish freighter Milos on the evening of 9 March. That night U-405 torpedoed the Norwegian freighter Bonneville while U-229 torpedoed the British freighter Nailsea Court and U-409 torpedoed the British escort oiler Rosewood and American ammunition ship Malantic.
The Flower-class corvettes HMS Campion and Mallow reinforced the convoy escort on 10 March, and the convoy reached Liverpool on 14 March. Only 76 of the 275 crewmen of the sunken ships were rescued.